Control optimisatlion of CO(2) cycles for medium temperature retail food refrigeration systems

This is the post-print version of the final paper published in International Journal of Refrigeration. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2009 Elsevier B.V.This paper describes a detailed procedure into the investigation of optimised control strategies for CO2 cycles in medium temperature retail food refrigeration systems. To achieve this objective, an integrated model was developed composing of a detailed condenser/gas cooler model, a simplified compressor model, an isenthalpic expansion process and constant evaporating temperature and superheating. The CO2 system can operate subcritically or transcritically depending on the ambient temperature. For a transcritical operation, a prediction can be made for optimised refrigerant discharge pressures from thermodynamic cycle calculations. When the system operates in the subcritical cycle, a floating discharge pressure control strategy is employed and the effect of different transitional ambient temperatures separating subcritical and transcritical cycles on system performance is investigated. The control strategy assumes variable compressor speed and adjustable air flow for the gas cooler/condenser to be modulated to achieve the constant cooling load requirement at different ambient conditions.DEFR

Evaluation of the energy impact of PCM tiles in an Airport Terminal Departure hall

Copyright @ 2013 CIBSEIn most past studies, passive PCM (phase change materials) systems have been tested for relatively small office spaces where the airflow is of minimal consequence in the overall energy consumption of the space. This paper on the other hand, reports on the application of PCM tiles on the floor of an Airport terminal space, similar to London Heathrow Terminal 5 departure hall, where in such large open spaces, the influence of airflow is crucial for the evaluation of the energy performance of AC units. In this paper, the evaluation of the energy performance of PCM tiles is obtained through a coupled simulation of TRNSYS and CFD. TRNSYS simulates the AC unit and PID control systems, while CFD is used to simulate the airflow and radiation inside the terminal space. The phase change process is simulated in CFD using an in-house developed model which considers hysteresis effects and the non-linear enthalpy-temperature relationship of PCMs. Although, a displacement ventilation (DV) system is actually employed at Heathrow Terminal 5, this study also compares the performance of the PCM tiles for a mixed ventilation (MV) system. Due to large computing times associated with CFD, discrete time-dependent scenarios under different UK weather conditions are used. The yearly energy demand is then determined through the heating/cooling degree day concept using base temperatures of 18 and 23 °C for HDD and CDD, respectively, similar to the comfort temperature range in the Terminal. The results show that the use of PCM tiles on the floor of the Terminal departure hall can lead to annual energy savings of around 3% for the DV system and 6% for the MV system, corresponding to 174 MWh/year and 379 MWh/year for the Terminal building.This work was funded by the UK Engineering and Physical Sciences Research Council (EPSRC), Grant No: EP/H004181/1

Energy aspects and ventilation of food retail buildings

Worldwide the food system is responsible for 33% of greenhouse gas emissions. It is estimated that by 2050, the total food production should be 70% more than current food production levels. In the UK, food chain is responsible for around 18% of final energy use and 20% of GHG emissions. Estimates indicate that energy savings of the order of 50% are achievable in food chains by appropriate technology changes in food production, processing, packaging, transportation, and consumption. Ventilation and infiltration account for a significant percentage of the energy use in food retail (supermarkets) and catering facilities such as restaurants and drink outlets. In addition, environmental conditions to maintain indoor air quality and comfort for the users with minimum energy use for such buildings are of primary importance for the business owners and designers. In particular, supermarkets and restaurants present design and operational challenges because the heating ventilation and air-conditioning system has some unique and diverse conditions that it must handle. This paper presents current information on energy use in food retail and catering facilities and continues by focusing on the role of ventilation strategies in food retail supermarkets. It presents the results of current studies in the UK where operational low carbon supermarkets are predicted to save 66% of CO2 emissions compared to a base case store. It shows that low energy ventilation strategies ranging from improved envelope air-tightness, natural ventilation components, reduction of specific fan power, ventilative cooling, novel refrigeration systems using CO2 combined with ventilation heat recovery and storage with phase change materials can lead to significant savings with attractive investment return

Analysis of energy use in crisp frying processes

Copyright @ 2010 Politecnico di Bari - BB PressWith increasing energy costs in industrial food frying processes it is essential to identify inefficiencies and minimise them. A way of achieving this is through the application of energy analysis and modelling techniques to characterise the process and investigate the interactions between the various operating and control parameters. The overall objective is to reduce energy consumption without compromising product throughput and quality. This paper provides a review of published work on heat and mass transfer in frying processes. Based on this, a simplified analysis of the key processes has been carried out using an energy balance model. The outputs of this model have been validated using data from an industrial crisp frying facility. The knowledge gained from this validation will be used to better understand and appreciate the energy flows in industrial frying processes and should lead to identification of losses and opportunities for energy recovery.The authors would like to acknowledge the financial support from the UK Engineering and Physical Sciences Research Council (EPSRC), Grant NO. EP/G059799/1, for this project as well as the input from the industrial collaborators and academic collaborators from the Universities of Newcastle and Northumbria

Modeling and Evaluation of the Thermohydraulic Performance of Compact Recuperative Heat Exchangers in Supercritical Carbon Dioxide Waste Heat to Power Conversion Systems

© 2021 The Author(s). Compact recuperative heat exchangers are critical components in supercritical carbon dioxide (sCO2) waste heat to power conversion systems. To investigate their thermohydraulic performance, a model based on the segmental design and the ε-NTU method has been developed. Four different types of heat exchanger have been considered: printed circuit heat exchanger with straight channels (PCHE-SC); printed circuit heat exchanger with zigzag channels (PCHE-ZC); microtube heat exchanger (MTHE); and microtube heat exchanger with separator sheets (MTHE-SS). The performance of the heat exchangers for different fluid mass flow rates, temperatures, and lengths was investigated in terms of Nusselt number, heat transfer coefficient, friction factor, pressure drop, heat transfer rate, entropy generation rate, and augmentation entropy generation number. Results show that these parameters significantly impact on the thermohydraulic performance of compact recuperative heat exchangers and their optimal design. For the same operating conditions and equal heat transfer rate, PCHE-ZC and MTHE-SS can have a significantly smaller size than PCHE-SC and MTHE. The augmentation entropy generation number also demonstrates the improved performance and compactness that can arise from zigzag channels and separator sheets, making them suitable for demanding high pressure and temperature applications such as sCO2 heat to power conversion systems.(i) The Engineering and Physical Sciences Research Council (EPSRC) of the UK under research grants EP/P004636/1, EP/V001795/1, and EP/ K011820/1 and (ii) the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 680599

Evaluation of Supermarket Energy Use and Emissions with Various Technology Options

In this paper, an operational supermarket in the UK has been selected to be modelled by the previously developed supermarket energy simulation software ‘SuperSIM’. Detailed information of the supermarket and model development procedures are explained. The model was previously validated through comparisons with site measurements of space air temperature and humidity and energy consumptions. It is therefore used to simulate, quantify and evaluate supermarket energy performance at various technology options in terms of heat recovery from refrigerant discharge, high efficiency condensers and evaporators and store locations etc

Modelling and performance evaluation of a low-temperature ammonia-water absorption refrigeration system

This paper presents the simulation of a low-temperature gas-fired ammonia-water absorption chiller for refrigeration applications. The model was developed as part of a research effort to investigate microturbine-based tri-generation systems for application in the food retail industry. The absorption chiller model was developed in the TRNSYS environment by integrating the main component models in the system and will form part of an overall TRNSYS-based supermarket model. The chiller model was validated against experimental results obtained on a 12 kW absorption chiller in the laboratory. The model was subsequently used to investigate the influence of important design and operating parameters on the performance of the chiller. © The Author 2009. Published by Oxford University Press. All rights reserved

Performance Analysis of Heat Exchangers and Integrated Supercritical CO<inf>2</inf> Brayton Cycle for Varying Heat Carrier, Cooling and Working Fluid Flow Rates

Data availability statement: All data used are in the paper but if any additional information is required it can be obtained by contacting the corresponding author.Copyright © 2022 The Author(s).. Supercritical CO2 power systems offer the potential of reduced system footprint and improved thermal efficiency, through the development and adoption of compact heat exchangers. Among these heat exchangers, the microtube, printed circuit, and plate heat exchangers are emerging as the most promising technologies for heat addition to the cycle, heat recuperation and heat rejection, respectively. To investigate the performance of supercritical CO2 recuperated Brayton cycle for heat to power conversion, simulation models of the heater, recuperator and cooler were developed using the distributed modeling approach and the ε-NTU method and then integrated with turbomachinery models to form the cycle model. The influences of flow rates of the heat carrier, cooling and working fluids on the heat exchanger performance and the integrated system were investigated. For the studied power system and under the off-design operating conditions, the net thermal efficiency of the cycle varies between 14.1% and 16.8%. Results show that increasing in the working fluid flow rate remains the net power output of the cycle but decreases the net cycle thermal efficiency, while increasing in the heat carrier fluid increases both, and the increase of cooling fluid increases the net power output but maintains the net thermal efficiency.(i) The Engineering and Physical Sciences Research Council (EPSRC) of the UK under research grants EP/P004636/1 – OPTEMIN, EP/V001795/1 – SCOTWOHR; (ii) the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 680599 – I-ThERM and Grant Agreement No. 101022831 – CO2OLHEAT

Design of a single-shaft compressor, generator, turbine for small-scale supercritical CO2 systems for waste heat to power conversion applications

Waste heat to power conversion is a promising approach to reduce the carbon intensity in industry and manufactured goods. In this framework, bottoming thermodynamic cycles using supercritical carbon dioxide as working fluid (sCO2) might be a suitable and efficient technology to consider especially for heat sources characterized by streams at high temperatures (>300°C). The compactness of sCO2 turbomachinery is one of the advantages of sCO2 systems over the conventional technologies; on the other hand, the reduced dimensions limits the bottom end of the power size achievable with such systems. The scarce amount of scientific and industrial literature for electrical power sizes between 50 and 100 kW further demonstrates this. The current research work summarizes the design procedure as weil as the technical and technological challenges involved in the design of a single-shaft compressor, generator, turbine unit (CGT) for a sCO2 system with a 50kWe nominal power output. First an overview of the High Temperature Heat To power Conversion facility (HT2C) under construction at Brunel University London will be presented. Then, highlights of the CGT design are presented in terms of structural and packaging aspects as weil as with regards to the ancillary lubrication, drainage and cooling loops